西藏群让蛇绿岩辉长岩SHRIMP锆石U-Pb年龄及地质意义

SHRIMP U-Pb Dating for the Gabbro in Qunrang Ophiolite,Tibet:The Geochronology Constraint for the Development of Eastern Tethys Basin

对雅鲁藏布江缝合带中段群让蛇绿岩中的辉长岩进行SHRIMP锆石U-Pb定年,得出加权平均年龄为125.6±0.88Ma(2σ,MSWD=1.9),即辉长岩结晶年龄。结合已有的关于雅鲁藏布江蛇绿岩形成年龄的报道,该结果表明群让地区特提斯洋海底扩张的时代与中段大竹卡、吉定地区一致;雅鲁藏布江西段与中段地区洋盆形成时代一致,但晚于东段发育时代;整个东特提斯洋盆发育时代存在东早西晚的特点。

Along the Yarlung-zangbo River valley, the Yarlung-zangbo Suture zone in Southern Tibet lasts along more than 2,000 kilometers from west to east, which is also the longest ophiolite belt in China. This ophiolite belt has drawn more and more attention from the geologists, due to its younger age and better reservation. But limited by the past analytical technology, the precise report about its formation time is singularly. And as product of the mag- matic event during the extension period of the middle ocean ridge, the gabbro in ophiolite possess a formation age corresponding to the extension time, and this age also represents the formation time of the ophiolite. In this paper, zircon SHRIMP U-Pb technique was applied to the gabbro in Qunrang ophiolite, west of the Yanglung Tangbo suture zone. A ^206pb/%23SU mean age 125.6 ± 0.88Ma（2σ, MSWD = 1.9） was obtained, which is the crystallization time of the gabbro. Compared with the former reported ophiolite ages in the Yanglung Tangbo suture zone （ 122.3 ± 2.4Ma for Xiugugabu ophiolite in the western part, 126 ± 1.5 Ma for the Dazhuka ophioilte and 123 ± 1.8Ma for the Jiding ophiolite in the middle part, while 162.9± 2.8Ma for Luobusha ophiolite in the eastern part） , this age suggests that the extension time of Tethys oceanic basin in Qunrang region was consistent with the formation time of the Dazhuq and Tiding oceanic basin in the middle part, which indicates that the development of the oceanic basin was isochronous both in the middle part and the western part while was later than the development of oceanic basin in the eastern part of the Yanglung Tangbo Suture zone. From the geochronology data, we can deduce that the open time of the eastern tethys basin was characterized by the east earlier and the west later.